Previously, displays, lighting systems and the like using an organic electroluminescence element (hereinafter referred to as an organic EL element as appropriate), which is an electroluminescence element, have been proposed. The organic EL element has a structure in which an organic layer having an organic light emitting region containing an organic light emitting material is held between a pair of electrodes.
However, mere employment of the above structure does not lead to completion of the organic EL element, and for example, at least one electrode should be capable of injecting electric charges (holes or electrons) into the organic layer and have transparency to light emitted at the organic light emitting region for extracting light to outside the element. In addition, for the organic layer, a material transporting electric charges injected from the electrode, recombining the electric charges to create an excited state, and emitting light when turning from the excited state to a ground state should be selected.
Therefore, materials for forming the organic EL element are extremely limited, and in most cases, there is no other choice but to use a material having a high volume resistivity for the transparent electrode and the organic layer.
Thus, there arises a problem that the current density at the organic layer varies depending on the position. A mechanism for this will be described below.
Generally, an electrode on the side through which light is extracted to outside the element is composed of a material having a higher volume resistivity, such as ITO, and the other electrode is composed of a material of which the volume resistivity is negligible compared with the electrode on the light extraction side. Thus, when resistance values in an infinite number of current paths in the organic EL element are considered, the length of passage over the electrode on the light extraction side on the paths may be examined.
If this examination is conducted, it will be apparent that a current path extending from a terminal portion of the electrode on the light extraction side through the electrode on the light extraction side to the other electrode by way of the organic layer at a position close to the terminal portion has a resistance value lower than that of a current path extending to the other electrode from the organic layer at a position distant from the terminal portion. That is, the current density in the organic layer at a position close to the terminal portion on the light extraction side is greater than the current density at a position distant from the terminal portion.
In this connection, the electrode on a side opposite to the light extraction side may be made of material having a volume resistivity higher than that of the electrode on the light extraction side. In this case, the position of the electrode on the light extraction side and the position of the other electrode may be mutually exchanged in the explanation described above.
As described above, it is difficult to equalize current densities at positions in the organic layer in the plane, and therefore, for example, the following phenomenon may occur.
Occurrence of Luminance Unevenness
Since there are locations where a large amount of current passes and where only a small amount of current passes, luminance unevenness occurs throughout the element. The luminance of the organic electroluminescence element becomes high as the amount of passing current increases (see, for example, Non-Patent Document 1), and therefore if there are locations where a large amount of current passes and where a small amount of current passes, a difference in luminance occurs between the locations, resulting in luminance unevenness.
For solving this problem, various techniques have been proposed.
For example, there is a prior art in which unloading portions (the terminal portions described above) for application of voltages are provided at many locations (see, for example, Patent Document 1). However, the size of an apparatus such as a portable terminal in which the organic EL element is incorporated is limited, and therefore the size of the organic EL element is also limited. Thus, provision of a large number of unloading portions as in the prior art is effective for solving the problem described above, but is extremely difficult to employ from a practical viewpoint. In addition, provision of a plurality of terminal portions leads to a problem that the proportion of the wirings for connecting the terminal portions to external drive circuit in the apparatus increases.
A prior art in which an auxiliary electrode made of material having a lower volume resistivity is placed on an electrode made of material having a higher volume resistivity is known. For example, a technique in which the auxiliary electrode is placed in one edge at a position where front and back sides are opposite to each other between a light emitting layer (the organic layer described above) and a transparent conductive film (the electrode described above) has been proposed (see, for example, Patent Document 2). This prior art is appropriately employed, but cannot completely solve the problem described above.
A prior art in which in-plane thickness fluctuations of layers constituting the organic layer are set to a predetermined value (see, for example, Patent Document 3), and a prior art in which the thickness of a light emitting layer in the organic layer (organic light emitting region) is adjusted at each position in the light emitting layer so that the luminance is uniform in the plane (see, for example, Patent Document 4) have been proposed. These prior arts can be appropriately employed, but it is extremely difficult from a practical viewpoint to change the thickness of each layer on a position-by-position basis in production of the organic EL element. In addition, for realizing the techniques, a special manufacturing method should be employed, and a manufacturing apparatus for realizing the manufacturing method should be fabricated.
A prior art relating to a line light source in which a light emitting region is segmented into a plurality of regions and the light emitting regions are connected in series has been proposed (see, for example, Patent Document 5). More specifically, it is a technique in which a plurality of thin film light emitting elements (light emitting regions) are connected in series to equalize the values of currents passing through the thin film light emitting elements, and the areas of the thin film light emitting elements are equalized to equalize current densities in the light emitting elements, whereby the luminances of the thin film light emitting elements are equalized.
In addition, in an inorganic electroluminescence element (hereinafter referred to as inorganic EL element as appropriate), which is an electroluminescence element, similar problems have arisen.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 5-315073 (claim 1, paragraph [0002])
[Patent Document 2] Japanese Laid-Open Utility Model Publication No. 5-20294 (claim 1)
[Patent Document 3] Japanese Laid-Open Patent Publication No. 11-339960 (claim 1)
[Patent Document 4] Japanese Laid-Open Patent Publication No. 11-40362 (claim 2, FIG. 1)
[Patent Document 5] Japanese Laid-Open Patent Publication No. 2000-173771 (paragraphs [0040]-[0046], paragraphs [0060]-[0065] FIG. 5, FIG. 7)
[Non-Patent Document 1] “Organic EL Elements and Front of Their Industrialization” supervised by Seizo Miyata, NTS CO., LTD., issued on 30 Nov., 1998, p. 46-47, FIG. 9